1. Field of the Invention
The present invention relates to a distortion correction circuit which is mounted on a television receiver or the like to correct image distortion.
2. Description of the Related Art
Conventionally, a raster on a fluorescent screen showed pincushion distortion in the deflection process of a color television receiver because the radius of curvature of a fluorescent screen is longer than the axial length from a deflection center to a fluorescent screen. This distortion is called pincushion distortion. Even though a uniform pincushion correction is applied to the entire screen, pincushion distortion still remains in the center part (hereinafter called inner pincushion distortion) because of the difference of distortion degree between the peripheral part and the center part of the screen. Such inner pincushion distortion which is a shrinkage in the center part is caused by the difference of a horizontal linearity between the upper/lower parts and the center part of the screen.
FIG. 7 shows a distortion correction circuit of voltage modulation type called a diode modulator circuit. A diode modulator circuit of negative system will be described below. In this circuit, distortion is corrected by using the negative potential at a point C.
In FIG. 7, pulses of the horizontal period are supplied to an input terminal 1 and supplied to the base of a horizontal output transistor Q1. A damper diode D1 is connected in parallel between the collector and the emitter of the horizontal output transistor Q1 so that the cathode of the former is connected with the collector of the latter. In addition, a resonant capacitor C2 is connected in parallel with the horizontal output transistor Q1. Furthermore, the combination of a horizontal deflection coil Ly, a linearity coil L1 and a damping resistor R1 which are connected in parallel with each other, and S-shape correction capacitors C4 and C5 serially connected are connected in parallel with that transistor Q1.
The collector of the horizontal output transistor Q1 is connected with a power supply terminal 2 through the primary winding T1 of a flayback transformer FBT so as to supply the power supply voltage VB. Additionally, a resonant capacitor C1 is connected between the collector of the horizontal output transistor Q1 and the reference potential point.
The emitter of the horizontal output transistor Q1 is connected with the reference potential point via a parallel circuit of a modulation diode D2 and a resonant capacitor C3, and at the same time via a modulation coil L3 and a modulation capacitor C6. A joint A between the S-shape correction capacitors C4 and C5 is connected with the reference potential point via a coil L2.
A joint B of the modulation coil L3 and the modulation capacitor C6 is connected with the reference potential point via a resistor R2 and the collector-emitter path of the transistor Q2. A parabolic wave generating circuit (not shown) which generates the parabolic wave voltage of the vertical period is connected with a terminal 3 which is connected with the base of the transistor Q2.
The above-mentioned circuit is a horizontal output circuit in which the damper diode D1 and the resonant capacitor C2 are connected in parallel with the horizontal output transistor Q1. In addition, in this circuit, a distortion correction circuit is connected with the peripheral part of the S-shape correction capacitors C4 and C5. The S-shape capacitor is divided into C4 and C5. A coil L2 is connected between the joint A of these capacitors C4 and C5 and the reference potential point. For the purpose of distortion correction, a distortion correction circuit consisting of a series circuit of the coil L3 and the capacitor C6 as well as the resonant capacitor C3 is connected in parallel between one terminal of the S-shape correction capacitor C5 (the emitter of Q1) and the reference potential point. The diode D2 maintains the negative voltage generated at the emitter of the transistor Q1 and guarantees the operation of the horizontal output transistor Q1.
In the horizontal scanning period, the horizontal deflection current flows through the horizontal output transistor Q1 or the damper diode D1. The current I1 flows from the S-shape correction capacitor C5 through the coil L2. The charge stored in the S-shape correction capacitors C4 and C5 flows to the horizontal deflection coil Ly through the coil L1 as a horizontal deflection current ly. Only the horizontal deflection current ly flows through the S-shape correction capacitor C4 on the horizontal deflection coil side. Both I1 and ly flow through the S-shape correction capacitor C5. The parabolic wave voltage of the vertical period is applied to the terminal 3 connected with the base of the transistor Q2 to modulate the terminal voltage Vm of the capacitor 6 parabolically. When the voltage Vm of the capacitor C6 is modulated parabolically with the vertical period, the S-shape capacitor voltage (Vc5+Vc4) as the power source of the horizontal deflection current ly is modulated as expressed by Vc5+Vc4=VB+Vm. In the above equation, Vc5 is the voltage across the S-shape correction capacitor C5, and Vc4 is the voltage across the S-shape correction capacitor C4. In this case, the capacity of the S-shape correction capacitor is set as C5&lt;&lt;C4, so Vc4 can be disregarded. That is, Vc5 is modulated parabolically with the vertical period by modulating Vm. Therefore, as shown in FIG. 8, the voltage of the horizontal period Vc5 modulated parabolically with the vertical period appears across the S-shape correction capacitor C5.
By using the voltage waveform modulated parabolically with the vertical period, inner pincushion distortion which occurs different horizontal linearities in the upper/lower and the center parts of the screen and occurs a shrinkage in the center part of the screen, is corrected to a uniform pincushion distortion as shown with the dotted line and the solid line in FIG. 9(a). Such corrected pincushion distortion is further corrected as shown in FIG. 9(b) by adjusting the parabolic wave by a DPC (Dynamic Pincushion Correction) circuit.
FIG. 10 shows the circuit diagram of a diode modulator circuit of positive system. Compared with the diode modulator circuit of the negative system shown in FIG. 7, the emitter of the horizontal output transistor Q1 is connected with the reference potential point and the diode D2 is connected serially in the same direction as that of the damper diode D1. In this circuit, the resonant capacitor C1 is removed between the collector of the horizontal output transistor Q1 and the reference potential point. In the circuit shown in FIG. 7, distortion is corrected by using the negative potential at the point C, but in the circuit shown in FIG. 10, such correction is performed by using the positive voltage at the point C.
However, the circuits shown in FIG. 7 or 10 are disadvantageous in the substrate area and the cost, because large parts such as coils and capacitors are employed. In addition, each time the inductance of a deflection yoke and the curvature of a picture tube change due to the model change, it is necessary to redesign the part constants, which is troublesome.